1. Field of the Invention
This invention relates to an image fixing device for fixing a toner image formed on an image holding member such as plain paper, photosensitive paper, etc. by the electrophotographic process, electrostatic recording process, magnetophotographic process and others.
2. Description of the Prior Art
A heating-fixing system has heretofore been adopted in a fixing device for fixing on a recording medium such as paper an unfixed image (hereinafter referred to as a toner image) formed by an image formation process such as electrophotographic process. In this type of heating and fixation, use has often been made of a device in which paper supporting a toner image thereon is conveyed between a heating roller provided with a heat source and a pressing roller urged against the heating roller.
In an image formation apparatus wherein such heating and fixation is effected, a fixing temperature of 150.degree. C. to 200.degree. C. has been required and therefore, a considerable time, i.e., about four minutes, has been necessary as the wait time after the main switch has been closed. Numerous technological inovations such as electrical control means and increased output of the heat source have been practised to reduce the wait time, but they have rather resulted in increased cost and other problems and complication of the apparatus.
Also, there are numerous problems in the construction of the heating and pressing rollers. For example, in the heating roller, heat distribution is liable to become nonuniform with respect to the axial direction thereof and heat dissipation is intense in the end portions of the roller, thus causing an increase in wait time. Further, during fixation, temperature rise may readily occur in the end portions because recording mediums of different widths are used and because the length of the roller is made longer than the width of the maximum size paper.
On the other hand, the roller provided on the pressing roller side can generally be grouped into three types, which have respective disadvantages as will hereinafter be described.
A first case is a fixing roller having a primer applied to a mandrel and a thick layer of silicone rubber or the like provided on the surface thereof, that is, a uniform elastic layer provided on a roller substrate. In this type of roller, oil or other additive must be added to reduce the elasticity of the elastic material such as rubber and as a result, the mechanical and physical characteristics of the elastic material are changed. Accordingly, it is impossible to reduce the hardness of the material so much while maintaining the characteristics thereof. Also, the pressure contact portion obtained relative to a supporting medium supporting thereon a toner image formed by the electrophotographic process or the like has been small and the amount of heat which can be imparted thereto per unit time has been small, and uneconomically, rubber or like material often used for the elastic layer has been required in a very great quantity and further, the wait time has become greater because the entire elastic layer must be heated, and this has increased the initial falling of the roller surface, thus causing insufficient fixation. An attempt to increase the number of heaters to prevent this has been uneconomical.
A second case is a fixing roller comprising a mandrel, a primer applied to the mandrel, and a porous member of silicone rubber provided thereon. A disadvantage peculiar to such roller is that it lacks the surface smoothness of the fixing roller and reduces the fixing ability and that where a parting agent is applied thereto, the porous member contains much of the parting agent and becomes ready to be deteriorated and thus, the durability thereof is inferior to rollers having solid or non-porus surfaces.
A third case is a multilayer construction in which two or three eleastic layers are provided to improve the oil-resisting property thereof. If multiple elastic layers are provided in this manner only for the purpose of improving the oil-resisting property, the cost thereof is greatly increased because a great deal of elastic material is used as previously described.
In the above-mentioned first and third cases, the plurality of layers or the thick rubber layer has led to a very great heat capacity, which in turn has resulted in an increased wait time and non-uniform temperature distribution throughout the length and therefore unsatisfactory fixation. This also holds true of the second case. Further, the use of much rubber material has led to high cost and poor durability.
As described above, in a fixing device having a heating roller and a pressing roller, there have been not only the problems of the individual constituents but also a combined problem caused by the required construction thereof and in particular, an increase in wait time has been caused.